In an internal combustion engine, the more delayed the closing timing of the intake valve, in other words, the more the closing timing of the intake valve approaches compression top dead center, the greater the amount of intake air pushed back from inside the combustion chamber to the inside of the intake port at the time of the compression stroke and therefore the less the amount of air sealed in the combustion chamber, that is, the amount of intake air fed into the combustion chamber. Therefore, by controlling the closing timing of the intake valve, the amount of intake air fed into the combustion chamber can be controlled.
Therefore, known in the art is a spark ignition type internal combustion engine provided with a variable timing mechanism able to control a closing timing of an intake valve and controlling the closing timing of the intake valve at the time of engine startup so that the amount of intake air required for startup is fed into the combustion chamber (for example, see Japanese Patent Publication (A) No. 2006-138299). In this internal combustion engine, when the engine is started up and the engine speed is rising, the engine speed is prevented from rising greatly exceeding a target idling speed at the time of engine startup, that is, the engine speed is prevented from overshooting as much as possible at the time of engine startup, by changing the closing timing of the intake valve so as to control the amount of intake air fed into the combustion chamber.
However, even if changing the closing timing of the intake valve in this way so as to control the amount of intake air fed into the combustion chamber, it is not possible to prevent overshoot of the engine speed at the time of engine startup.
That is, in the above-mentioned internal combustion engine, at the time of engine startup, it is necessary to reduce the amount of intake air fed into the combustion chamber to a small amount so as to prevent the engine speed from overshooting. For this, at the time of engine startup, it is necessary to delay the closing timing of the intake valve considerably. However, if delaying the closing timing of the intake valve, the timing at which the compression action is actually started becomes delayed, so the actual compression ratio ends up falling and ignition ends up becoming impossible.
Therefore, in the above-mentioned internal combustion engine, to raise the actual compression ratio for ignition, it is necessary to advance the closing timing of the intake valve beyond the optimum closing timing for preventing overshoot. As a result, the engine speed inevitably overshoots at the time of engine startup. Therefore, as explained above, even if changing the closing timing of the intake valve to control the amount of intake air fed into the combustion chamber, it is not possible to prevent overshoot of the engine speed at the time of engine startup.